A hidden single is a digit with only one place left in a house, even if that cell still shows other candidates.
Each Jigsaw region must hold 1 to 9 once. Its shape is irregular, so trace it carefully: the cells that see a digit are not the ones a box would have.
Worked examples
2 positions from real Jigsaw puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the arrows show what rules what out.
One place left in a row
Look at row 6. The digit 6 must go somewhere in this row. Where can it go?
One place left in a region
The lit region is a house of nine cells, like a box, but it can wander across rows and columns.
Where it fits
MediumHidden Single is a Medium technique: Medium Jigsaw puzzles are the first to need it.
The Classic Hidden Single lesson teaches it on a plain 9×9 board. The Jigsaw overview explains the rule and the levels.
Hidden Single in the other styles: Sudoku X, Thermo, Windoku, Arrow, Kropki.
Try it on a Medium Jigsaw puzzle. Next Step points it out when it is the move.